1. Field of the Invention
This invention pertains to apparatus for converting thermal energy into mechanical energy, and more particularly to apparatus for converting the heat energy of burning fuel in an external combustion engine into rotary mechanical motion.
2. Description of the Prior Art
The Stirling engine, which has been well-known for over 100 years, possesses several characteristics that make it very attractive as a prime source of mechanical power. Particularly in multi-cylinder engines, these characteristics include quietness, high thermal and mechanical efficiencies, and the ability to use many different fuels. Further, the normal heat-to-mechanical energy conversion of the Stirling cycle is reversible, so that input of mechanical energy enables a Stirling engine to operate as a refrigerator or heat pump.
The flexibility with which the cylinders of a multi-cylinder engine can be arranged has not been fully exploited in the prior art Stirling engines. In a recent multi-cylinder engine design, the cylinders are grouped in banks of adjacent in-line cylinders supplied with heat from a single heat source. Each bank of cylinders requires a linear-to-rotary motion converter, such as a crankshaft (U-drive arrangement). To maintain the timing between the banks, the crankshafts must be connected with timing gears or similar devices. Multiple crankshafts and multiple timing devices are undesirable because they add weight and expense to the engine.
A solution to the U-drive multi-crankshaft problem is shown in U.S. Pat. No. 3,200,602, wherein the two banks of cylinders converge toward a single crankshaft (V-arrangement). However, the penalty is that a single heat source may not be possible for supplying both banks of cylinders without becoming unduly large and wasteful of heat. Some designs utilize a swash plate to convert the reciprocating motion of the pistons in two adjacent banks to rotary motion. However, swash plates are failure-prone, expensive to manufacture, and introduce unncecssary complexities into the Stirling engine.
Despite its inherent advantages, the multi-cylinder Stirling engine has not proven a commercial success. One reason is that, since each piston is double acting, both ends of the cylinders in which the pistons reciprocate must be sealed against leakage of the working fluid.
In the prior art Stirling engines, a piston rod extends through one end of the cylinder to the components that convert the piston reciprocating motion to rotary motion. Thus, the joint between the piston rod and cylinder end wall must be well sealed. Operating pressures of the working fluid in the cylinders may reach 2,500 pounds per square inch, making sealing extremely difficult. Adequate sealing has proven to be a major problem, as explained in the book "Stirling Engines" by G. Walker, Clarendon Press, pages 360-383, and in the articles "Stirling Auto Engine", Popular Science, January, 1983, page 50, and "Stirling-Cycle Engine Promises Low Emissions Without Add-ons", Popular Science, February, 1973, page 72. Although these publications discuss the sealing problem, it is apparent that no adequate solution has been found.
Another shortcoming of prior art Stirling engines, as exemplified by the publications mentioned previously and by U.S. Pat. Nos. 3,200,602 and 3,379,026, is that the cool ends of the cylinders are located between the hot ends and the crankcase. In that location, the mass of the engine tends to retain heat that flows thereto from other parts of the engine. To maintain proper operation of the engine, the heat must be removed by a large, costly and complicated cooling system incorporating a water-filled radiator and cooling jacket. In addition, heat in the crankcase region is undesirable because it exacerbates the piston rod sealing problem.
Thus, a need exists for a single crankshaft multi-cylinder Stirling engine that eliminates piston rod sealing problems and that employs a single heat source to serve a multiplicity of cylinders having thermally isolated cool ends.